Introduction
With the development of vitrification technology, the survival and intact cell rates are increasing and the implantation, clinical pregnancy and live birth rates of the vitrification are similar to the fresh.1-3 Embryo cryopreservation makes it possible to conserve surplus embryos for further use, and increases cumulative live-birth rates with one oocyte retrieval operation. The advantages of frozen-thawed embryo transfer (FET) are not only decreasing the rate of ovarian hyperstimulation syndrome (OHSS), but also providing a more favorable intrauterine environment for embryo implantation and placentation by avoiding the supraphysiologic condition that occurs after ovarian stimulation.4-7 Hence, the proportion of FET is increasing in assisted reproductive technologies. The factors affecting the outcome of FET are female age, embryo characteristics, endometrial preparation protocol, number of embryos transferred and so on.8, 9
Currently, the two most common procedures to select embryos for FET differ in the duration of the post-thawed culture: one relies upon the observation of blastomere survival after thawing, requiring a short culture; another is dependent on the observation of blastomere proliferation, requiring a long culture, generally overnight. Some studies indicated that cleavage stage embryos with low blastomere number had significantly decreased developmental potential and pregnancy rate,10-12 and extended culture of these embryos to blastocysts can resulted in a similar clinical pregnancy rate as that of blastocysts derived from embryos with ≥ 7 blastomeres.12, 13 The influence of post-thawed culture for about 18 h on the outcome of FET is still controversial,12, 14, 15 and the reasons may be related to the different sample sizes, inclusion criteria, embryo freezing methods, embryo quality and age distribution of patients in different studies. Previous studies focused on the comparison between short culture and long culture, while ignoring the effect of age and embryo quality on implantation and live birth rates. In this study, a retrospective analysis improved sample size (containing 9832 FET cycles), age distribution (divided into three groups: <35, 35-39 and > 39) and embryo quality (good-quality embryos) to evaluate the influence of long post-thaw culture period on pregnancy and clinical outcomes.